Apply gofmt -w across the codebase (struct field comment realignment
only — no semantic changes) and silence two errcheck warnings on
fmt.Sscanf / fmt.Fprintf return values in internal/router/discovery
with explicit `_, _ =` discards. Required so `make check` is green
before tagging v0.1.0.
Add custom MarshalJSON/UnmarshalJSON on Content using string type discriminant
("text", "tool_call", "tool_result", "thinking"). Add json tags to Message.